This invention relates to a semiconductive device manufacturing method capable of easily manufacturing high frequency elements.
In order to get a transistor for use in a bipolar IC suitable for high frequency, in general, it is necessary to settle its cut-off frequency f.sub.1 -high. Such transistor for high frequency has conventionally been produced by the following process:
Step (a): As shown in FIG. 1(A), an n-type embedded region 2 is formed on the surface of a p-type silicon substrate 1 by the well-known selective diffusion method, an n-type epitaxial film 3 is thereafter provided on the surfaces of the p-type silicon substrate 1 and the n-type embedded region 2 by the well-known epitaxial method and an oxide film 4 is formed by oxidizing the surface of the n-type epitaxial film 3.
Step (b): As shown in FIG. 1(B), the oxide film 4 is selectively removed at positions thereof wherein inactive base regions for lowering base resistance are provided to thereby form windows 5A and 5B by well-known photolithography method.
Step (c): As shown in FIG. 1(C), p-type inactive base regions 6A and 6B are selectively formed by doping boron from the windows 5A and 5B and the windows are thereafter covered by the oxide film 5 again.
Step (d): As shown in FIG. 1(D), the oxide film 4 is selectively removed at a predetermined position thereof for providing an active base region in to thereby form a window 7. Through the window 7, the p-type inactive base regions 6A and 6B as well as the n-type collector region 3 are exposed.
Step (e): As shown in FiG. 1(E), a p-type active base region 8 is formed by doping boron from the window 7. The p-type base region 8 is so formed to communicate with the inactive base regions 6A and 6B and to be lower and shallower in its impurity concentration than the inactive base regions 6A and 6B. The window 7 is thereafter covered by the oxide film 4 again.
Step (f): As shown in FIG. 1(F), the oxide film 4 is removed at predetermined places for providing an emitter region and a collector ohmic region to thereby form windows 9 and 10.
Step (g): As shown in FIG. 1(G), an n-type emitter region 11 and a collector ohmic region 12 are formed by doping phosphorus or arsenic from the windows 9 and 10. The windows 9 and 10 are thereafter covered by the oxide film 4 again.
Step (h): As shown in FIG. 1(H), the oxide film 4 is removed at portions thereof for providing an emitter electrode, base electrode and collector electrode to thereby form windows 13, 14 and 15.
By sequently mounting metallic material like aluminum on the individual windows 13, 14 and 15, an npn transistor is finished.
In such conventional manufacturing method, the window 9 for making the emitter region 11 is so designed to be wider than the window 13 for thereafter making the emitter electrode taking into consideration imposing error produced upon imposing a mask by the photolithography method. This means that the actual area of the emitter must be much larger than the truely effective one.
Further, although positions of the emitter region and base region with respect to each other are determined by accuracy of superposing a mask, fine working has been difficult for the same reason. Such conventional method does not permit formation of a shallow emitter region and a thin base width. Beside these, the base region is formed over two times, which leads to increase of man hours and poor characteristic reproducibility due to existence of errors.
For those reasons given above, according to the conventional method, it has been difficult to settle cut-off frequency high and a high-frequency transistor has not been realized.